Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for characterizing fractures based on seismic data; more particularly, to mechanisms and techniques for solving azimuth angle ambiguity thus improving amplitude variation with offset and azimuth (AVOAz) technique by incorporating geological information and/or using far-offset (i.e., larger incidence angles) data.
Discussion of the Background
Hydrocarbons (i.e., crude oil and natural gas) may be found in layers of rock deep beneath the surface of the earth or seafloor. Prospecting in search of hydrocarbon resources is an ongoing process driven by continuously increasing worldwide demand. Seismic surveys are a prospecting tool used to generate a profile (image) of underground geophysical structures. This profile provides information indicating whether hydrocarbons are likely present. Obtaining high-resolution images of underground geophysical structures based on seismic data is, therefore, desirable.
During a marine seismic survey, as illustrated in a vertical plane (yz) view in FIG. 1A, a vessel 110 tows plural detectors 112, which are disposed along a cable 114. Those skilled in the art use the term “streamer” for cable 114 and its corresponding detectors 112. Vessel 110 may tow plural streamers 116 in the horizontal (xy) plane. Streamer 116 is towed at a substantially constant depth z1 relative to the water surface 118. However, streamers may be towed slanted (i.e., to form a constant angle) with respect to the water surface, or may have a curved profile as described, for example, in U.S. Pat. No. 8,593,904, the entire content of which is incorporated herein by reference.
Vessel 110 (or another vessel) may also tow a seismic source 120 configured to generate acoustic waves 122a. Acoustic waves 122a propagate downward and penetrate the seafloor 124. When encountering a reflecting structure 126 (reflector R), an acoustic wave is at least partially reflected. Reflected acoustic waves 122b and 122c propagate upward. For simplicity, FIG. 1A shows only two paths 122a corresponding to the source-emitted acoustic waves. Reflected acoustic wave 122b is recorded by various detectors 112 (recorded signals are called traces), while reflected wave 122c passes detectors 112 and is reflected back at the water surface 118 (the interface between the water and air serving as a quasi-perfect reflector to mirror acoustic waves). The wave reflected by the water surface may then be detected as illustrated by wave 122d in FIG. 1A. Wave 122d is normally referred to as a ghost wave because it is due to a water-surface reflection traveling downward, rather than upward directly from inside the explored structure.
FIG. 1B illustrates the geometry of a wave 122A at R in FIG. 1A (considered horizontal for the relevant reflection area). The geometry is characterized by the incidence/reflection acute angle θ and the azimuth angle ϕ in FIG. 1b. The incidence/reflection acute angle θ is formed by the incoming or reflected wave and vertical direction, which is considered normal to the reflector's surface. The azimuth angle ϕ is the angle between the reflected wave's projection in the horizontal plane and a reference direction (x-North).
The detectors record amplitude versus time series, known as traces, which are processed to generate a reflectivity image of the underground structure 124 and, in particular, the location of reflectors 126 and the nature of rock layers, which experts associate with the likelihood of oil and/or gas presence. Although FIG. 1A illustrates a marine streamer seismic acquisition system, a water-bottom seismic or a land seismic acquisition system is similar to the marine seismic acquisition system in the sense that the water-bottom seismic system has seismic sensors distributed over the water bottom surface, while the land seismic data acquisition system has seismic sensors distributed over land surface, and seismic sources (e.g., vibrators) are moved by trucks from place to place to generate seismic waves.
Remotely detecting information about fractures and the stress field is an important objective in the development of unconventional and tight hydrocarbon reservoirs. Fractures are defined as cracks in rock that typically have apertures of a few millimeters or less. Fractures and stress cause the earth to become anisotropic, which is seismically observable. Fluid-filled fractures respond somewhat like a spring to a seismic wave as it passes across them if the seismic wave is perpendicular to the fracture. A seismic wave traveling parallel to fractures does not encounter the same spring-like behavior. Therefore, seismic waves traveling in different directions measure different rock velocities when a layer contains fractures.
The fractures' presence and their orientation can be inferred by observing the P-wave seismic amplitude variation with offset and azimuth (AVOAz). Conventional seismic data processing methods are unable to uniquely determine a fracture's orientation, e.g., yielding two possible solutions 90° apart. This issue is well-known in the case of near-offset AVOAz inversion, but it is also true for far-offset approximation. In both cases, azimuthal ambiguity leads to biases in other anisotropy-related parameter estimates.
Thus, there is a need to develop new seismic data processing methods that overcome the fracture orientation ambiguity issue.